The method of making a positive choke device

ABSTRACT

Disclosed is a positive choke device having bore liner of silicon carbide. Also disclosed is a method of fabricating flow beans and the like having bore coatings of corrosion resistant materials having a hardness of at least 2,700 on the Knoop scale without machining of hard materials.

United States Patent McClure et al. 1 Aug. 29, 1972 [54] THE METHOD OFMAKING A [56] References Cited POSITIVE CHOKE DEVICE UNITED STATESPATENTS [72] lnventors: James II. McClure, Dallas; James l Mccmry,Richardson both of 2,640,503 6/1953 Millngan ..138/ 141 T ex himaryExaminer-John F. Campbell [73] Assignee: Materials TechnologyCorporation, Assistant Examiner Donald p Rooney Dallas Attorney.lack A.Kanz [22] Filed: May 11, 1970 ABSTRACT 211 App]. No.: 35,976

Disclosed is a positive choke device having bore lmer of siliconcarbide. Also disclosed is a method of [52] US. CL ...............29/l57C, 29/157 B, 138/141, fabricating flow beans and the like having bore184/ 5 2 coatings of corrosion resistant materials having a hard- [51]Int. Cl.....B2ld 53/00, B2lk 29/00, B23p 15/26 ness of at least 2,700 onthe Knoop Scale without [58] Field of Search ..29/l57 C, 157 B; 184/52;

machining of hard materials.

SCIaims, 3 DrawingFigures PATENTEU M1829 m2 F'ICLZ INVENTOR JAMES H.MCCLURE JAMES W MCCRARY CURRENT SOURCE w mzm THE METHOD OF MAKING APOSITIVE CHOKE DEVICE This invention relates to flow regulating devices.More particularly it relates to devices commonly known as positivechokes which have an orifice of fixed dimension and are secured within aline of high pressure fluid to control the rate of flow of fluid throughthe line.

in the petroleum producing industry it is often required to accuratelycontrol the rate of flow of crude petroleum and the like under highpressures through a particular conduit. For this purpose constrictiondevices, commonly referred to as positive chokes or flow beans, areinserted in the conduit. Such devices have a predetermined orificepassing therethrough which is determinative of the rate of flow of fluidunder fixed conditions of temperature and pressure. Accordingly, therate of flow of petroleum through a conduit can be controlled byaffixing within the conduit a choke or flow bean of reduced diameterwhich restricts the flow through the conduit and regulates the flow offluid therethrough.

Since the rate of flow of fluid through the choke is determined by thesize of the aperture, precise control over the size of the aperture inthe choke must be maintained at all times. However, ordinary crudepetroleum may contain large amounts of hydrogen sulfide gas or othercorrosive materials which attack and corrode conventional materials suchas stainless steel. Furthermore, crude petroleum often contains largeconcentrations of abrasive materials such as sand and the like. Sincethe crude petroleum may be forced through the choke at pressures as highas 10,000 psi, the choke may be seriously abraded by foreign materialscontained in the petroleum stream. Conventional materials, such asstainless steel and the like, are seriously corroded and abraded whenused under these conditions and rapidly lose their effectiveness forprecisely controlling the rate of fluid flow therethrough.

Attempts have been made to overcome these problems by placing liners ofcorrosion resistant materials within the choke orifice. However, it isdifficult to accurately machine suitable materials to the dimensionsrequired, since most corrosion and abrasion resistant materials arequite brittle.

In accordance with this invention, a positive choke is provided whichhas a liner of extremely hard, corrosion and abrasion resistant materialin the orifice. The unique choke is formed without mechanical milling ormachining of hard or brittle materials. The liner, however, has theuniform dimensions required to effectively operate as a positive choke.The liner is formed by chemical vapor deposition of silicon carbide on agraphite rod. The graphite is removed leaving a dense, impermeable linerof silicon carbide which is then secured within the choke orifice.

Because of the unique characteristics of silicon carbide and the mannerin which the liner of the invention is fabricated, the choke device ofthis invention is substantially immune to attack by corrosive materialscommonly encountered in crude petroleum streams, and is alsosubstantially immune to abrasion by the abrasive materials contained inthe crude petroleum. Accordingly, the liner protects the orifice of thechoke and prevents deterioration thereof.

A particular advantage and feature of the invention is the provision ofa positive choke which is resistant to both corrosion and abrasion. Thechoke may be formed with the required uniform dimensions withoutmachining of hard materials. Other features and advantages of theinvention will become more readily understood from the followingdetailed description taken in connection with the appended claims andattached drawing in which:

FIG. 1 is a sectional view of one embodiment of a positive choke made inaccordance with the invention;

FIG. 2 is a schematic drawing of the process system for producing theliner of the invention; and

FIG. 3 is a perspective view of a graphite rod coated with siliconcarbide in accordance with the process described with reference to FIG.2.

In accordance with the present invention a liner is inserted within theorifice of the choke or flow bean to protect the orifice from bothcorrosion and abrasion by foreign materials contained in the crudepetroleum stream. However it should be noted that in accordance withthis invention the liner is silicon carbide produced by vapor depositionof silicon carbide on a rod of graphite. Accordingly, the siliconcarbide liner of the flow bean is not machined in any manner to producethe precision orifice required. inserts of silicon carbide havingprecise internal dimensions are produced by depositing silicon carbideon precision ground rods of graphite as described hereinafter withreference to FIGS. 2 and 3.

In order for a positive choke device to precisely control the flow offluid therethrough, the bore passing through the device must be ofsubstantially uniform diameter. Furthermore, the bore must besubstantially smooth to avoid causing unnecessary turbulance in thechoke.

While it is not difiicult to form smooth uniform bores in conventionalmaterials such as stainless steel, it is quite difficult to bore uniformholes in extremely hard materials. However, in order to prevent abrasionof the walls of the bore, the walls must be at least as hard as theabrasive materials contained in the fluid stream. Furthermore, the borewalls must be extremely dense to prevent abrasive particles frombecoming embedded in the bore walls and causing abrasion thereof as thefluid is forced through the choke under high pressures.

in accordance with the method of this invention, a bore liner isproduced which has a surface hardness of at least 2,700 on the Knoopscale and has smooth uniform internal dimensions without machining,milling or polishing of hard materials. The liner is produced as acoating on a relatively soft rod which is then removed leaving only thecoating. The coating is then inserted as a liner in the bore of thechoke device. Since the internal dimensions of the liner conformprecisely to the outer dimensions of the rod upon which the coating wasformed, the bore of the liner is made smooth and uniform by depositingthe coating on a precision ground smooth graphite rod.

in the preferred practice of the invention silicon carbide is depositedon graphite rods since graphite is quite readily machined to the desireddimensions and silicon carbide may be formed on graphite without excessdifficulty. However, since the reaction occurs at elevated temperatures,care must be taken to select a material for the rod which has acoefficient of thermal expansion which closely approximates that ofsilicon carbide to avoid cracking of the coating. For this reason a highgrade, high expansivity graphite, such as Speer Grade 9345 is preferred.This grade graphite has a thermal coefficient or expansion of about 5 Xcm/cm/C. Silicon carbide produced by the method described herein has athermal coefficient or expansion of 4.5 X 10' cm/cmC.

Silicon carbide may be formed by several known methods. ln the preferredmethod of the invention a dense impermeable coating of silicon carbideis formed by reacting silicon halide with a hydrocarbon containing gason a heated graphite surface. While it will be understood that variousreactant combinations may be used to deposit silicon carbide, thepreferred process is described herein with reference to FIG. 2.

The apparatus of FIG. 2 includes a scalable deposition chamber 20comprised of upper portion 21 and lower portion 22. The upper and lowerportions are removably secured together by conventional means such asbolts 23, clamps or the like. Chamber 20 has an exhaust port 24connected to exhaust line 25 which is in turn connected to aconventional vacuum pump or the like for removing gases from thedeposition chamber.

A rotatable table 26 is mounted on a shaft 27 which passes transverselythrough the bottom of the deposition chamber 20 and is adapted forrotation by conventional means. Rotatable table 26 is preferablyconstructed of a relatively inert material such as graphite or the likewhile the deposition chamber 20 may be constructed of stainless steel orany other suitable material.

A radiant heater 28 is secured below rotatable table 26 andinterconnected to a suitable power source 29 for heating materials inthe deposition chamber. Reactants are injected into the reaction chamberthrough lines 30 and 31 by way of control valves 32 and 33,respectively, and into nozzle 34. Nozzle 34 projects into the upperportion of the deposition chamber and directs the reactants toward thesurface of the rotatable table 26.

Precision ground graphite rods 35 of uniform diameter are positionedsubstantially vertically on table 26 for rotation therewith and directlybelow the nozzle 34. The deposition chamber 20 is then closed andsealed.

The sealed chamber is evacuated by withdrawing the atmosphere therefromthrough line 25 and the chamber refilled with dry hydrogen through inlet30, valve 32 and nozzle 34. When the chamber 20 is filled with dryhydrogen, the exhaust port 24 is opened and hydrogen allowed to flowthrough the chamber 20 at the rate of about 25 to about 50 liters perminute.

With hydrogen flowing through the chamber at essentially atmosphericpressure heater 28 is activated by passing current therethrough fromcurrent source 29. The graphite rods 35 are heated to a temperaturebetween about 1,000 C and about 1,400 C and maintained at thistemperature under flowing hydrogen for about to about 30 minutes toassure complete cleaning and outgasing of the graphite rods. Throughoutthe cleaning and following deposition process, rotatable table 26 isrotated at a rate of about 1 to 15 rpm. It is therefore uniformly heatedby radiant energy from the radiant heater 28 and all graphite rods 35are maintained at a relatively constant temperature.

After the graphite rods 35 have been thoroughly cleaned as describedabove, the flow of hydrogen through the deposition chamber is stoppedand hydrogen containing silicon tetrachloride (SiCL) substitutedtherefor. Simultaneously with the introduction of hydrogen and silicontetrachloride containing gas, a mixture of hydrogen and toluene isintroduced through line 31, valve 33 and into nozzle 34.

In the preferred practice of the invention, the composition of thereactant gas entering chamber 20 through nozzle 34 is approximately 2 toabout l0 mole percent silicon tetrachloride, about 0.3 to about l.5 molepercent toluene, and the remainder hydrogen, the total gas flow throughthe system being about 50 liters per minute to about 100 liters perminute. It will be understood that within the gas composition rangesgiven an approximate stoichiometric relationship should be maintainedwith respect to silicon and carbon in the reactant gas stream. Thereforeit will be apparent that the mole ratio of SiCl, to C H CH, should beapproximately 7:1. Throughout the deposition process the temperature ofthe graphite rods is maintained at a constant temperature between aboutl,000 C and about l,400 C. Under these conditions a uniform densecoating of silicon carbide is formed on the surface of the graphite rodsat rates up to 40 mils per hour.

When the desired thickness of silicon carbide is formed on the surfaceof the graphite rods 35, the flow of reactants through nozzle 34 isstopped and pure dry hydrogen substituted therefor. The flow of currentthrough radiant heater 28 is then stopped and the rods 35 allowed tocool to substantially room temperature in flowing hydrogen. Thedeposition chamber 20 is then flushed with nitrogen and the coated rodsremoved.

Upon removal from the deposition chamber the coated rods are cut intosuitable lengths such as illustrated in FIG. 3. The rod producedcomprises a graphite rod 35 having a coating 40 of dense impermeablesilicon carbide adherently bonded to the surface thereof. lt will beobserved, however, that vapor deposited silicon carbide formed asdescribed above forms a coating 40 which conforms precisely to theexternal dimensions of the rod 35.

The coating 40 may be recovered and removed from the rod 35 bychemically removing the graphite rod. A solvent must be selected, ofcourse, which dissolves the graphite without affecting silicon carbide.A solution comprising 1 part BNO, and 9 parts H by volume, has beenfound suitable for this purpose. In the preferred practice of theinvention, part of the graphite is first removed by boring a holeaxially through the center of the rod. in this manner approximately 80percent of the graphite rod can be quickly removed and the remaininggraphite rapidly dissolved in the HNO :H,SO solution.

Since the HNO :H SO solution dissolves graphite but does not attacksilicon carbide, the rod -35 is completely removed leaving only thecoating 40. Furthermore, since the coating formed a mirror image of thesurface upon which it was deposited, the bore through coating 40 isprecisely the same dimensions as the surface upon which it was formed.

Vapor deposited silicon carbide coatings 40 produced as described abovehave been found to have a density of 3.22 grams per cubic centimeter anda surface hardness of 2,740 on the Knoop scale, using a 100 gram weight.The material is also found to have an excellent resistance to oxidationup to temperatures as high as l,500 C and has a compressive strength of119 X psi and essentially zero porosity with a coating thickness of atleast 0.005 inch. Accordingly, liners having the above characteristicsare resistant to attack by any corrosive materials contained in fluidstreams such as crude petroleum and the like. Furthermore, because ofthe extreme surface hardness and smoothness of the surface produced,abrasive materials contained in a fluid stream passing through such aliner have little effect on the silicon carbide surface.

A positive choke constructed according to the invention is illustratedin FIG. 1. The choke comprises a stud having a hex head 10 with athreaded shank 11. A bore 12 passes axially through the full length ofthe choke. Liner 40 is positioned within the bore 12 and securedtherewithin as by brazing or with a suitable adhesive or otherconventional means. A suitable adhesive for securing the liner 40 withinthe bore of the bean has been found to be an adhesive epoxy cementmanufactured by the Armstrong Company of Warsaw, lndianna marketed underthe trade name of Armstrong A-2 Adhesive.

Conventional positive chokes are generally fabricated from stainlesssteel or the like, have bore dimensions ranging from 6/64 inch up to 1inch, and are usually about 1 inch to about 6 inches long. Chokes madein accordance with this invention can be made from conventional chokematerials with oversized bores into which the liners are inserted. Theliners 40 can be made to have any desired internal diameter by selectinggraphite rods of the desired diameter. Accordingly, chokes made inaccordance with this inven tion may be readily substituted forconventional chokes. Furthermore, liners having any desired internaldiameter may be produced without machining hard materials by depositingsilicon carbide on a graphite rod having outside dimensions of thedesired bore. Since graphite is comparatively soft and readily machined,rods may be precision ground to any desired size. Consequently, theliners, which conform to the configuration of the rod, can be formedwhich have smooth, uniform bores with variations in internal diameter ofless than i- 1 mil throughout the length of the bore.

While the invention has been described with particular reference to apositive choke, other similar devices may be fabricated by the samemethod. For example,

flow control is sometimes maintained using a master bean of a firstinternal diameter and having a threaded socket at one open end. Flowbeans of the general configuration illustrated in FIG. 1 may then besecured within the socket to reduce the aperture of the choke. Flowbeans having larger or smaller apertures may thus be readily substitutedto change the flow characteristics of the choke. Such flow beans mayreadily be fabricated in accordance with this invention as describedabove with reference to positive chokes whi i t e t he invention hasbeen described with specific reference to specific embodiments thereof,it is to be understood that the forms of the invention shown anddescribed in detail are to be taken as preferred embodiments of same,and that various changes and modifications may be resorted to withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

We claim:

I. The method of producing a positive choke device comprising the stepsof:

a. forming a rod of graphite having external dimensions conforming tothe desired dimensions of the bore of the positive choke to be produced;

b. heating said graphite rod to a temperature between about 1,000 C and1,400" C;

c. depositing a coating of dense impermeable silicon carbide on thesurface of said graphite rod;

d. removing the graphite rod from the silicon carbide leaving a tubularbody of silicon carbide having internal dimensions conforming to thedesired bore dimensions of the positive choke to be produced; and

e. securing said tubular body of silicon carbide within the bore of apositive choke device.

2. The method set forth in claim 1 wherein said coating of siliconcarbide is deposited on said graphite rod by reacting a hydrocarbon anda silicon halide on the surface of said graphite rod.

3. The method set forth in claim 2 wherein said hydrocarbon is tolueneand said silicon halide is silicon tetrachloride.

4. The method set forth in claim 3 wherein said silicon tetrachlorideand said toluene are entrained in a hydrogen stream in a ratio ofapproximately 7. l.

5. The method set forth in claim 1 wherein said graphite rod is removedfrom said silicon carbide coating by boring a hole axially through thecenter of said graphite rod and dissolving the remaining graphite in asolution comprising about 1 part HNO, and 9 parts H by volume.

2. The method set forth in claim 1 wherein said coating of siliconcarbide is deposited on said graphite rod by reacting a hydrocarbon anda silicon halide on the surface of said graphite rod.
 3. The method setforth in claim 2 wherein said hydrocarbon is toluene and said siliconhalide is silicon tetrachloride.
 4. The method set forth in claim 3wherein said silicon tetrachloride and said toluene are entrained in ahydrogen stream in a ratio of approximately 7:1.
 5. The method set forthin claim 1 wherein said graphite rod is removed from said siliconcarbide coating by boring a hole axially through the center of saidgraphite rod and dissolving the remaining graphite in a solutioncomprising about 1 part HNO3 and 9 parts H2SO4 by volume.